For many purposes in agriculture and related endeavors it is desired to treat plants with exogenous chemical substances of various kinds. Many exogenous chemical substances are applied to foliage (i.e., leaves and other non-woody above-ground parts) of a plant, and have a site of action in the plant either close to or remote from the locus of application. Such substances are referred to herein as foliar-applied exogenous chemical substances.
Typically, when an exogenous chemical substance is applied to foliage by plant treatment processes known in the art, only a small portion of the amount applied reaches sites of action in the plant where a desired biological activity of the exogenous chemical substance can be usefully expressed. It is therefore a major desideratum in agriculture and related endeavors to enhance the efficiency of delivery of foliar-applied exogenous chemical substances to their sites of action in plants, and thereby to enhance the biological effectiveness of the exogenous chemical substance for the purpose for which the exogenous chemical substance is used.
Application to foliage of an exogenous chemical substance by processes known in the art does not universally result in inefficient delivery to sites of action. In some situations such processes provide excellent biological effectiveness, even at a low use rate of the exogenous chemical substance. In other situations the same processes, using the same rate of the exogenous chemical substance, provide inadequate biological effectiveness. Thus, these processes are inconsistent in the result they provide, or they cannot be relied upon to provide the desired result.
A problem is that it is seldom possible to identify in advance those situations where good biological effectiveness will be obtained, partly because so many factors influence efficiency of delivery. These factors include weather (temperature, relative humidity, daylength, cloudiness, precipitation, wind, etc.) preceding, during and following application, soil conditions (fertility, aeration, etc.), plant growth stage, health and physiological status, equipment-related inaccuracies in application, and other factors. Therefore, to help ensure reliable or consistent biological effectiveness of a foliar-applied exogenous chemical substance, the user typically applies the substance at a higher rate than truly necessary in the majority of situations.
Variability in biological effectiveness in field conditions is an especially troublesome problem in the case of exogenous chemical substances that are acids, and are typically formulated as water-soluble salts in which the exogenous chemical substance is present in an anionic form. Sometimes by converting such acid substances to esters, this variability can be moderated; however, in many cases esters show reduced biological effectiveness, for example due to inadequate conversion back to the parent acid once inside the treated plant. There remains a strong need for enhanced biological effectiveness, and enhanced reliability of biological effectiveness, of foliar-applied exogenous chemical substances, particularly anionic exogenous chemical substances.
An "anionic exogenous chemical substance" herein is an exogenous chemical substance whose molecular structure includes one or more acid, or proton-donating, sites, and is therefore capable of forming an anion in the presence of a proton acceptor. The term embraces substances that are zwitterionic. In describing an exogenous chemical substance as "anionic" herein, it is not implied that the exogenous chemical substance is necessarily in anionic form or that it is dissociated.
Benefits of a process providing greater reliability of biological effectiveness include an ability to reduce rates of application of exogenous chemical substances without sacrificing consistency of biological effectiveness. Pressures felt by the agricultural industry to reduce pesticide, particularly herbicide, usage are well evidenced by symposia on the subject, such as that held in 1993 by the Weed Science Society of America and documented in Weed Technology 8, 331-386 (1994). Reduced use rates bring rewards not only environmentally but also economically, as the cost per unit area treated decreases.
Foliar-applied exogenous chemical substances have frequently been applied together with amphiphilic materials, particularly amphiphilic surface-active agents, otherwise known as surfactants. Surfactants can influence biological effectiveness of a foliar-applied exogenous chemical substance in numerous ways.
When a dilute aqueous composition of an exogenous chemical substance is applied to foliage by conventional hydraulic spraying, the presence of surfactant in the dilute aqueous composition can alter the size distribution of the spray droplets, typically increasing the percentage of spray volume in the form of small droplets and reducing the percentage of spray volume in the form of large droplets. As smaller droplets have lower momentum than larger droplets, these smaller droplets are less likely to rebound from a foliar surface and consequently are more likely to be retained on that surface. Spray retention can also be facilitated by adhesion between surfactant molecules in a spray droplet and the foliar surface, which in most plants is waxy and hydrophobic. This adhesion reduces not only rebound but also run-off of spray droplets from the foliar surface. Surfactants also tend to increase the area of contact between a spray droplet and a foliar surface, and in many cases enhance penetration of an exogenous chemical substance from the droplet into and through cuticles of leaves to reach internal leaf tissues.
A class of surfactant that has been of particular interest for enhancing delivery of foliar-applied exogenous chemical substances to the interior of plant foliage is characterized by a hydrophobic moiety comprising three or more silicon atoms linked by oxygen atoms to form a trisiloxane or polysiloxane group. Such surfactants, herein called "siloxane surfactants", are a subclass of organosilicone surfactants and are exemplified by o-methoxypolyethoxypropylheptamethyltrisiloxane, sold for example by the OSi group of Witco Corporation as Silwet.RTM. L-77.
Silwet.RTM. L-77 has been reported to enhance foliar absorption of a wide range of exogenous chemical substances, including the herbicide glyphosate, by plants, this enhancement having been attributed at least in part to infiltration of stomata and other microscopic apertures in the foliar surface. This enhanced absorption commonly leads to enhanced biological effectiveness. However, on some plant species, at least under certain conditions, such enhanced biological effectiveness is not exhibited; indeed the presence of Silwet.RTM. L-77 has been found in many cases to cause antagonism of biological effectiveness of an exogenous chemical substance. As used herein, "antagonism" refers to a decrease in biological (such as herbicidal) effectiveness of an exogenous chemical substance (such as a herbicide) when a material (such as Silwet.RTM. L-77) is used in combination with the exogenous chemical substance.
Particularly in the case of herbicides, the risk of antagonism provides a disincentive to use siloxane surfactants such as Silwet.RTM. L-77, because multiple weed species are typically treated in the same field and the surfactant is likely to prove antagonistic for at least some of the weed species present.
Another problem, well known in the art, that has discouraged extensive use of siloxane surfactants such as Silwet.RTM. L-77 is hydrolytic instability of the trisiloxane or polysiloxane moiety in an aqueous medium. Cleavage of silicon-oxygen bonds in the trisiloxane or polysiloxane moiety results in degradation products which are ineffective as stomatal infiltrants and ineffective in enhancing biological effectiveness of foliar-applied exogenous chemical substances. Although hydrolysis occurs in the dilute aqueous compositions prepared by the end-user for application to foliage, typically these compositions are prepared immediately before use and the time available for hydrolysis is correspondingly short. More significant is the problem of hydrolysis in an aqueous concentrate composition, which for commercial acceptability must have a shelf-life of several months to several years.
This is a particular problem with compositions comprising water-soluble exogenous chemical substances, because these substances are most conveniently and economically formulated as aqueous concentrates. Even more particularly, it is a problem with anionic exogenous chemical substances, as these are often formulated at pH&lt;7, i.e., at acid pH, and at such pH levels hydrolysis of siloxane surfactants tends to be accelerated. For example, glyphosate is most typically formulated as a mono-salt which, in aqueous solution, gives a pH of around 4.
It would be a major advance in the art to provide a water-based concentrate composition of a water-soluble exogenous chemical substance such as a glyphosate salt containing a siloxane surfactant that is effective as a stomatal infiltrant, yet wherein the siloxane surfactant has acceptable long-term chemical stability.
Glyphosate (N-phosphonomethylglycine) in its strict sense is an acid compound, but the word "glyphosate" is herein used in a less restrictive sense, except where the context dictates otherwise, to encompass not only glyphosate acid but also salts, adducts and esters thereof, and compounds which are converted to glyphosate in plant tissues or which otherwise provide glyphosate ions. In most commercial formulations of glyphosate, the glyphosate is present as a water-soluble salt. In this respect, glyphosate is typical of most exogenous chemical substances that are acids or that form anions.
Herbicidal salts of glyphosate are disclosed, for example, in U.S. Pat. No. 3,799,758 to Franz, U.S. Pat. No. 3,853,530 to Franz, U.S. Pat. No. 4,140,513 to Prill, U.S. Pat. No. 4,315,765 to Large, U.S. Pat. No. 4,405,531 to Franz, U.S. Pat. No.4,481,026 to Prisbylla and U.S. Pat. No.4,507,250 to Bakel. In most of the salts disclosed, the counterion to glyphosate anion is a relatively low molecular weight, non-amphiphilic cation. Typical of such salts are alkali metal, for example sodium and potassium, salts; ammonium salt; and salts having an ammonium, sulfonium or sulfoxonium cation substituted with 1-3 organic groups containing in total 1-6 carbon atoms, for example dimethylammonium, isopropylammonium, ethanolammonium and trimethylsulfonium salts.
Commercial formulations of glyphosate salts include, for example, Roundup.RTM. brand, Accord.RTM. brand, Roundup.RTM. Ultra brand and Roundup.RTM. Xtra brand herbicides of Monsanto Company, which contain the isopropylammonium salt, Roundup.RTM. Dry brand and Rival.RTM. brand herbicides of Monsanto Company, which contain the ammonium salt, Roundup.RTM. Geoforce brand herbicide of Monsanto Company, which contains the sodium salt, and Touchdown.RTM. brand herbicide of Zeneca, which contains the trimethylsulfonium salt.
Australian Patent No. 658150 discloses liquid aqueous concentrate compositions comprising a salt of glyphosate, a silicone copolymer wetting agent exemplified by the siloxane surfactant Silwet.RTM. L-77, and an amphoteric surfactant.